Fix tokenizer extra_special_tokens crash and stabilize transformers

feat: Add percentage progress to downloading status in UI and extension

feat: Add download status polling to UI and Firefox extension

feat: Switch models to use Gemma 4 from unsloth collection

feat: Add support for switching models and the 27b-4a backend

feat: Add Global Saliency Calculation Mode

feat: Auto-recompute text in extension when saving settings

feat: add FlowRead Entire Page functionality to Firefox extension

docs: Add local and HF deployment options and Firefox extension instructions